Marantz 140 Problem

snugglebear

New Member
I've discovered a problem with my 140 amp. I recently replaced the caps and de-oxed the input pots and cleaned up all the connectors to the amp boards. The amp plays just fine. I did notice that the right VU meter does not track the same as the left meter when a mono signal is being amplified. So, I opened up the amp and checked the DC balance and bias and went to check the VU meter settings. I followed the procedure to adjust the meters and I cannot get the 9 volts RMS from the right channel to adjust the meter per the instructions. The most output I can get is about 6.5 volts RMS. I also noticed that the bias adjustment on the right channel is almost turned fully clockwise. The measured bias voltage of the right amp is about 8.5 mV per the instructions. The left is set for the same voltage but the bias adjust on the left is about dead center.

Obviously I've got something that is weak in this amp but the problem is I am not sure where to start looking. My first guess would be the output transistors which I replaced with new onsemi's but that didn't change the output levels. My next guess would be transistor H709 or maybe the H713/H714 pair. Physically, there aren't any burn marks or discolorations on the amp board and nothing is getting hot so I'm kind of stumped. I could use a little help getting this fixed up. I'm reaching out to the experts (all of you) for a little advice what to check next.

Chris
 
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